The responses of both cloned and heterogeneous T cell populations to mitogens and specific antigenic stimuli were found to require the participation of accessory or antigen presenting cells which express Ia (I region associated) determinants. A series of T cell clones which recognized the same antigenic molecule in association with a given I-E molecule were studied for their susceptibility to inhibition by a panel of 15 different anti I-E monoclonal antibodies specific for determinants on the same I-E molecule. The results demonstrated that different T cell clones appear to recognize antigen in association with distinct determinants or conformations on the same I-E molecule. The responses of T cell clones to the mitogen Mycoplasma arthritidis supernatant (MAS) were analyzed. It was determined that approximately 15% of the clones tested responded to MAS when presented in the context of I-E bearing antigen presenting cells, and that this reactivity was independent of the primary specificity of MHC restriction of those clones to conventional antigens. The ability of different Ia expressing populations to present to cloned T cells was evaluated. All of the clones tested were able to respond to adherent cell-containing irradiated stimulating cells or to mitomycin treated purified resting B cell populations. In contrast, only a subpopulation of cloned T cells was responsive to LPS activated B cell blasts, in spite of the enhanced Ia expression by these B cell blasts. These findings indicate that the signals required for T cell activation may vary among T cell clones, and that distinct Ia bearing antigen presenting cells may be competent for presentation to different T cells. Additional studies employing cloned Ia+ B cell lymphomas and hybridomas also indicate a heterogeneity among T cells in their responsiveness to these populations.